In continuous ink jet printing, print head devices are constructed to: (i) form ink streams which break up within a drop charging region into drop streams of uniformly sized and spaced ink drops, (ii) selectively impose electrical charge on some drops at stream breakup in accord with an information signal and (iii) deflect charged drops to a predetermined trajectory, which can be either a print trajectory or a "caught" trajectory. The "caught", non-printing drops are returned to the ink reservoir and recirculated to the print head.
Most prior art print head assemblies employ drop charge electrodes adjacent the drop formation point and drop deflection electrodes that are separate of, and spaced downstream from, the charge electrode. U.S. Pat. No. 3,656,171 discloses a prior art technique wherein a single electrode structure extends from the drop breakup region to the drop catch region and functions as both the drop charging and drop deflection electrode. U.S. Pat. No. 4,636,808 discloses an improvement over the '171 patent wherein a "non-extended" charge and deflection electrode that has a path length dimension of only about 6 drop spacings or less, provides combined drop charging and drop deflecting functions. The '808 patent approach has the significant advantage of removing the electrodes from contact with charged ink drops while still eliminating the need for a separate deflection electrode structure.
The '808 patent system for combining the functions of drop charging and deflection into a single, short electrode works quite well. However, we have found that the drop deflections which are produced by that system are not uniform and vary based on the charge or non-charge selection as to subsequent drops. This can be understood by conceptually separating the charging and deflection functions of the combined charge/deflection plate. The upper region of that plate, adjacent to the break off point, produces the drop charging. The lower region of that plate will have several print or catch drops in front of it and serves as a deflection electrode. To produce a print drop (e.g. uncharged) the charge voltage on the combined charge/deflection plate is switched to 0 volts for 1 stimulation cycle. This switching off of the charge voltage also turns off the deflection field in front of the lower region of the combined plate and the deflections of the drops in front of the lower part of the charge plate are thus reduced (in comparison to the deflections imparted when the plate is charging a drop).
The trajectories of charged drops in the '808 system therefore can range between a maximum deflection (where the combined plate is charging all drops during a drop passage past the plate) and a minimum deflection (where the plate is energized only to charge that drop and thereafter grounded). While such non-uniformities in deflected drop trajectories of the '808 patent system do not prevent good printing operations, they do present overall printer design constraints that are undesirable. For example, the range of possible trajectories necessitates a relatively high charge electrode voltage, i.e. reduces the charging voltage range over which proper printing is possible.